1. Field of the Invention
The present invention relates to an integrated dielectric substrate, and more particularly to a dielectric substrate equipped with a dissipating plate, for mounting an exothermic element (e. g., a high-output transistor or integrated circuit) used for power modules in mobile communication tools, etc.
2. Description of the Related Art
FIG. 8A shows the structure of an integrated dielectric substrate. FIG. 8B is a fragmentary cross sectional view taken along the line A--A in FIG. 8A. The integrated dielectric substrate is composed of: a dielectric substrate 1; metallic blocks 2; a dissipating plate 3; exothermic elements 5 generating a large amount of heat, such as high-output transistors (power transistors) or integrated circuits (ICs); and connecting terminals 7.
As is shown in FIGS. 8A and 8B, the dielectric substrate 1 is mounted on the top face of the dissipating plate 3. The dielectric substrate 1 has through holes 4 corresponding to the number of the exothermic elements 5 to be mounted on the dielectric substrate 1. The metallic blocks 2 are inserted into the corresponding through holes 4 and the bottom face of each metallic block 2 is connected to the dissipating plate 3. The exothermic elements 5 are each mounted on the top face of a respective metallic block 2 according to a one-to-one correspondence. The top-face area of each metallic block 2 is larger than that of the corresponding exothermic element 5. Each of the through holes 4 defines a space between the corresponding metallic block 2 and the substrate 1.
On the surface of the dielectric substrate 1, circuit patterns (not shown in the figure) and land patterns 8 are formed, and also, devices (not shown in the figure) other than the exothermic elements 5 are loaded. The exothermic elements 5 and the land patterns 8 are connected by wires 10 so that the exothermic elements 5 achieve continuity with the predetermined connecting positions of the circuit patterns via the land patterns 8 and wires 10.
The dissipating plate 3 exhibits a high heat conductivity and has terminal portions 11 extending from the ends of the dielectric substrate 1. The heat generated by the exothermic elements 5 is conducted to the dissipating plate 3 through the metallic blocks 2, and then, dissipated outward through the terminal portions 11.
Connecting terminals 7 are used for fixing the above integrated dielectric substrate to a mother board (not shown in the figures).
As is mentioned above, in the above integrated dielectric substrate, one through hole 4 is made for receiving each exothermic element 5. The size of each through hole 4 is required to be large enough to insert the corresponding metallic block 2, whose top-face area is greater than that of the exothermic elements 5, with a space between the dielectric substrate 1 and the metallic block 2. Thus the size of the through holes 4 is considerably larger than the top face of the exothermic elements 5.
The size and the number of the through holes 4 made in the dielectric substrate 1 are predetermined and cannot be decreased. Therefore, when the size of the dielectric substrate 1 is reduced for achieving a compact integrated dielectric substrate, the proportional area of the through holes 4 in the dielectric substrate 1 significantly increases, resulting in the following problem: the minimum area necessary for forming the circuit patterns and for loading devices other than the exothermic elements 5 cannot be ensured, and thus achievement of compact integrated dielectric substrate becomes difficult.
In addition, when the dielectric substrate 1 has a multi-layer structure in which circuit patterns are formed in each layer, the area available for forming the circuit patterns becomes small in each layer because every layer has the through holes 4. Thus, to ensure a sufficient area for forming the circuit patterns, the dielectric substrate 1 must be large, undesirably resulting in a large-sized integrated dielectric substrate.